Tractors often are equipped with extra wheels, so-called dual wheels, when working in fields where it is desirable to distribute the weight of the tractor over a large surface so that the earth in the wheel tracks does not become too heavily compressed. A further advantage of dual wheels is that tractors driving through muddy fields do not get stuck so easily, the tractive force being transferred via a larger wheel surface.
Quite a number of systems for releasable coupling of an extra wheel to an original wheel are known. In Danish patent specification No. 139.617, for instance, a coupling system is described consisting of a number of inter-acting locking cams and clamps in the form of knee-action mechanisms distributed along the periphery of the wheel, the mechanisms being fastened to the original rim and to the dual rim, respectively. However, these known coupling systems are complicated mechanisms calling for extremely accurate alignment of the dual wheel and the original wheel before assembly in the axial direction of the wheels. As there is often question of very large and heavy dual wheels, the alignment process obviously calls for either additional equipment, which means increased cost, or more operators. Furthermore, the tractive force is transferred from the original wheel to the dual wheel solely through the relatively sensitive coupling arrangements, and it has become apparent that the coupling known from Danish patent specification No. 139,617, in the case of relatively large wheels, can only be used for short periods before having to be replaced or repaired.
Furthermore, from the Danish patent No. 136,889 a tightening mechanism is known for securing extra wheels on the original wheels of a vehicle. The tightening mechanism consists of at least three angle levers rotatably placed in a hub device releasably fastened to the extra rim, and furthermore at their free ends the angle levers are provided with a hook arranged to engage into locking cams secured to the original rim, in which cams holes are provided corresponding to the hooks, when the levers are rotated about their mentioned center of rotation by turning a hand wheel helically engaged in said angle levers.
So, by turning the hand wheel the hooks at the free ends of the angle levers are pressed radially into the holes of the locking cams while fastening the dual wheel to the original wheel.
This construction also calls for an exact alignment as the hooks of the angle levers have to be opposite the holes in the locking cams on the original rim, and thus the use of extra equipment or more operators for this work will be required. Moreover, it is a substantial disadvantage that the tractive force has to be transferred from the original wheel to the dual wheel through the locking cams and the rotatably embedded angle levers. Therefore, the shakings or jolts occurring during the driving will cause loosening of the angle levers and the locking cams which had been tightened by the hand wheel.